Method of reducing pressure and controlling flow rate of a fluid under high pressure

ABSTRACT

The pressure of a fluid under high pressure can be reduced as the flow rate of said fluid is kept at an arbitrarily selected value by passing said fluid through a circuit comprising a long tubing or a reduction nozzle or series of nozzles and introducing a flow rate controlling medium into said circuit.

United States Patent 1151 3,646,950

Takehisa et a], Mar. 7, 1972 [54] METHOD OF REDUCING PRESSURE 58 FieldofSearch ..137/13, 14; 62/48 AND CONTROLLING FLOW RATE OF A FLUID UNDERHIGH PRESSURE [561 Refcrenm CM [72] lnventors: Masaaki Takehisa;Waichiro Kawakamit- UNITED STATES PATENTS Takisaki'shh' 3,006,35410/1961 Sommeretal ..137/13 3,092,981 6/1963 Begeman etal ..62/48 xJapan 3,389,714 6/1968 Hughes et a1. 137/13 [73] Assignee: Japan AtomicEnergy Research Institute,

Tokyo, Japan Primary Examiner-Robert G. Nilson Altorneyl(urt Kelman [22]Filed: Apr. 1, 1969 I 21 Appl. No.: 812,247 1 ABSTRACT The pressure of afluid under high pressure can be reduced as [30] Foreign Applicationpriority n the flow rate of said fluid is kept at an arbitrarilyselected value by passing said fluid through a circuit comprising a longApr. 12, 1968 Japan ..43/240l0 tubing or a reduction nozzle or Series fnozzles and introducing a flow rate controlling medium into saidcircuit.

[52] US. Cl ..l37/13, 137/14, 62/48 [51] Int. Cl ..Fl7d 1/16 16 Claims,1 Drawing Figure METHOD OF REDUCING PRESSURE AND CONTROLLING FLOW RATEOF A FLUID UNDER HIGH PRESSURE BACKGROUND OF THE INVENTION Thisinvention relates to a method for reducing the pressure of a fluid,including gas, liquid and slurry under high pressure and preciselycontrolling its flow rate at a predetermined value in a high-pressureapparatus.

It is often necessary to reduce the pressure of a fluid under highpressure and to hold its flow rate constant, for instance, when a fluidcontaining a reaction product is taken out of a high-pressure reactionvessel. It is however, very difficult to do this, and this problem hasnot been technically solved until now. As the means for reducing thepressure of a high-pressure fluid, methods using reducing valves or longtubing have been employed. However, the operation of valves isdifficult; and when dust or solid foreign materials are in the fluid,when a slurry is to be treated, or lhen the fluid is a gas which iseasily liquefied or solidified upon pressure reduction such as ethyleneor carbon dioxide, pressure reduction by the use of a reducing valve isalmost impossible. Further, when solid foreign materials are in thefluid, not only is the life of the valve shortened owing to abrasion,but pressure reduction itself is extremely difflcult. On the other hand,when long tubing is used, the relation between pressure and flow rate isdefinitely determined for a given diameter and length of the tubing,since the pressure of the flowing fluid is reduced by frictionresistance when the fluid flows inside the tubing. Therefore, in methodsusing long tubing, the flow rate and pressure can not be arbitrarilyvaried.

The purpose of this invention is to provide a novel method for pressurereduction and flow-rate control of a fluid under high pressure, in whichthe above-mentioned defects of the prior art have been eliminated.

SUMMARY OF THE INVENTION In this invention precise control of thepressure and the flow rate of a high-pressure fluid is achieved bypassing the. highpressure fluid (including slurry) through a long tubingor a circuit containing reducing nozzles (hereinafter called pressurereduction circuit"), introducing a flow rate controlling medium into thecircuit at the midpoint thereof, and regulating the amount of mediumsupplied. The flow rate controlling medium (hereinafter simply calledmedium) is generally a liquid which is immiscible with the fluid thepressure of which is to be reduced. But a liquid which is miscible withthe fluid can also be used if the liquid or medium can be separated fromthe fluid afterward or if it is unnecessary to separate the medium fromthe fluid.

The method of this invention can be applied to pressure reduction andflow rate control of a fluid under high pressure which comprises a gas,a liquid, a gas and liquid mixture, two or more immiscible liquids, aslurry, or a gas, liquid and solid mixture.

That is to say, this invention provides a method for pressure reductionand fluid-flow rate control in a high-pressure fluid system selectedfrom a gaseous phase system, a liquid phase system, a gas-liquidtwo-phase system, slurry, a system comprising two or more immiscibleliquids, or a gas-liquid-solid system. It comprises passing said fluidthrough a pressurereduction circuit comprising a long tubing or areduction nozzle or a series of nozzles, introducing a flow ratecontrolling medium into said pressure-reduction circuit, andregulatingthe flow rate of the medium.

The method of this invention is advantageously utilized in cases inwhich polymerization is carried out in a high-pressure gaseous or liquidphase and the polymerization product has to be continuously taken out ofthe high-pressure reactor to the ambient space. The invention is nowexplained in detail, the attached drawing being referred to.

BRIEF EXPLANATION OF DRAWING and Fl stands for flow rate indicator.

DETAILED EXPLANATION OF THE INVENTION In the apparatus schematicallyshown in the drawing, a fluid, that is, a gas, a liquid, a gas-liquidmixture, a slurry (the dispersed solids may be a reaction productproduced in a high pressure reactor), a mixture of two or moreimmiscible liquids, or. a gasliquid-solid system, is pressurized to apredetermined pressure by means of a diaphragm-type twostep compressor 1having a maximum compressing pressure of 400 kg./cm. and a maximum flowrate of IO m. (at a temperature of 0 C. and at atmospheric pressure/hr.and is charged into high-pressure vessel 2 having a volume of 7 litersdesigned for a normal operating pressure of 400 kg./cm. The pressureinside this system is maintained constant by means of pneumatic controlvalve (PCV) which is commercially available; In order to reduce thepressure of the fluid as it' flows at a predetermined rate, a flow rate.controlling medium is introduced into the pressure reduction circuit atmixing port 3 by means of plunger pump 6, which has a capacity of 400kg./cm. at the maximum delivery pressure and liter (NTP)/hr. at themaximum flow rate, so that the fluid and the medium may flow together inthe circuit. The pressure reduction circuit is a long tubing of l-5 mm.inside diameter. Plunger pump 6 is a commercially availableflow-rate-variable one equipped with stepless variator. Member 7 is astorage tank for the flow rate controlling medium. The pressure at whichsaid controlling medium is introduced must be at least the same as orsomewhat higher than that inside the high-pressure circuit so as toprevent backflow from the pressurereduction circuit. Alternatively, apreliminary pressure-reducing means represented by 4a may be providedupstream of the circuit, and the medium may be introduced into the fluidafter the pressure has'been reduced 21 little. Otherwise, a check valvemay be provided at the inlet of the mixing port 3. The mixture ofhigh-pressure fluid and flow rate controlling medium is passed throughpressure-reduction circuit 4 whereby its pressure is reduced. The designof the pressure reduction circuit and all the above-mentioned system canbe easily worked out by the skilled in the art by consideringthe purposeof the pressure reduction. For instance, when the pressure to be reducedis that of a gas that is easily liquefied or solidified, such asethylene or carbon dioxide, the pressure drop should be kept low enoughto insure that the gas will not solidify. In this case, if the pressureis reduced substantially in the preliminary pressure-reduction circuit4a, then relatively little reduction is required in the reductioncircuit 4 and less energy will be consumed by pump 6. When the pressureof a slurry is reduced, in order to prevent the hazard of clogging thebest policy is to employ a thin slurry and it is desirable to reduce thepressure as little as possible in the preliminary reduction circuit 4a.

Design, dimension and materials of the pressure-reduction circuit isdetermined case by case by considering the properties and flow rate ofthe fluid the pressure of which is to be reduced, the properties of theflow rate controlling medium, the capacity of the plunger pump 6 and thepressure difference (absolute value of pressure reduction).

The flow rate of a high-pressure fluid may be increased or decreased bydecreasing or increasing the amount of the medium to be introduced intothe pressure-reduction circuit per unit time. If varied degrees ofpressure reduction are to be obtained, varied reduced pressuresindicated by pressure indicators PI-3, 4, 5 and 6 are achieved by takingout the fluid from respectively corresponding portions in the reductioncircuit as indicated in the drawing.

When the fluid the pressure of which is to be reduced, is a gas, forexample, and a'liquid is used as the flow rate conto pump 6, the flowrate of the fluid (gas) can be automatically controlled. I

If for some reason it is necessary to keep the inside of separator at asuperatmospheric pressure, it is, of course, possible to reduce thepressure of the fluid to the superatmospheric pressure.

Now the invention will be illustrated by way of the preferredembodiments or working examples. It is to be understood, however, thatthe scope of the invention is not restricted to these embodiments only.

EXAMPLE 1 Pressure-reduction and flow rate control of nitrogen.

An experiment on pressure-reduction and flow rate control of nitrogenwas carried out using an apparatus in accordance with the invention asrepresented by the attached drawing. The result is shown in Table 1.

Water was used as the flow rate controlling medium. By varying theamount of the water introduced into the circuit from 30 to 100liters/hr., the flow rate of nitrogen was controlled between lO-Om.(NTP)/hr., as its pressure was reduced. As the pressure-reductioncircuit, a stainless steel tubing of 2 mm. inside diameter and 90 m.length was used.

EXAMPLE 2 Pressure reduction and flow rate control of ethylene.

it is impossible to reduce the pressure of ethylene: by a valve becauseethylene liquefies by rapid temperature drop upon adiabatic expansion.

By using the apparatus represented by the drawing in accordance withthis invention, pressure reduction and flow rate control was carriedout. The result is shown in Table 2. A stainless steel tubing of 2 mm.inside diameter and 85 m. length was used as the pressure reductioncircuit, a stainless steel tube of 2 mm. inside diameter and 20 m.length as the preliminary reduction circuit 4a, and methanol was used asthe flow-rate-controlling medium. Ethylene dissolves in methanol underpressure. But when the pressure is removed, the ethylene issubstantially separated from methanol, only a small amount (about 1percent by weight) of ethylene remains dissolved in the methanol.

By varying the amount of the medium introduced into thepressure-reduction circuit, a wide range of pressure reduction and flowrate of ethylene were achieved.

EXAMPLE 3 Pressure reduction and flow rate control of carbon dioxide.

When the pressure of gaseous carbon dioxide is to be reduced by means ofvalves, the flow rate being kept constant, dry ice forms and clogs thenozzle or tube and thus pressure reduction is impossible.

Pressure-reduction and flow rate control of high-pressure carbon dioxidewas carried out using the same apparatus as used in Example 2. The flowrate control conditions were the same as in Example 2. The result isshown in Table 4. Carbon dioxide dissolves in methanol. But it is easilyseparated from the solvent in the same way as in Example 2.

Pressure and flow rate can be controlled over a wide range.

EXAMPLE 4 Pressure reduction and flow rate control ofethylenepolyethylene slurry.

In the apparatus of the attached drawing, an autoclave having a capacityof 500 ml. equipped with an agitator was installed as the high-pressurevessel 2. Six grams of polyethylene powder the particle size of which isabout 100 mesh (ASTM) was put in the autoclave and more ethylene wascharged therein until the inside pressure reached 400 kg./cm. Thepolyethylene powder existed dispersed in the liquefied ethylene in theautoclave and its concentration in the slurry was about 3 percent byweight. Methanol as the flow rate controlling medium was introduced at arate of 1.5 liter/min. (indicated by Fl-2) by means of pump 6. Thus allthe polyethylene powder in the autoclave could be recovered atatmospheric pressure without any loss.

In this case, there was no preliminary reduction circuit; the pressurereduction circuit was a long stainless steel tubing of 2 mm. insidediameter and 85 m. length. The flow rate of ethylene was 5 m."( NTP)/hr.(indicated by FM It is impossible to reduce the pressure of a slurrylike this using an ordinary needle valve or stop valve.

The used methanol was separated from ethylene in separator 5. Theseparated methanol contained about 1 percent by weight of ethylene atatmospheric pressure and was recirculated to storage 7 to be used as theflow rate controlling medium again.

EXAMPLE 5 Pressure-reduction and flow rate control of a slurrycomprising ethylene, methanol and polyethylene powder.

.The apparatus represented by the attached drawingwas used instead ofthe high-pressure vessel (2), and a reactor for producing polyethylenewas installed. A reaction system comprising a mixture of ethylene andmethanol (50:50 by weight) in the liquid state was continuously suppliedinto the reactor. The reactor was placed in an irradiation zone of acobalt-60 of 10 Curies. Such irradiation apparatus is well known tothose who are skilled in the art.

The reaction mixture (ethylene-methanol mixture) was continuouslysupplied into the reactor so as to maintain the iriside pressure at 400kg./cm. and the residence time of the mixture in the reactor constant.The slurry comprising the ethylene-methanol mixture and polyethylenepowder that was produced in the reactor was continuously taken out tothe ambient space by passing it through the pressure-reduction circuitand introducing water as the flow rate controlling medium into thecircuit at a flow rate of 1.5 liter/min. (F [-2).

It is possible to use methanol as the flow rate controlling medium, andthis is advantageous because the separated methanol (containing about 1percent by weight of ethylene at atmospheric pressure) can be used as acomponent of the reaction mixture or the flow rate controlling medium.

EXAMPLE 6 Pressure-reduction and flow rate dichlorodifluoromethane-watersystem.

A high-pressure liquid-liquid system consisting of liquefieddichlorodifluoromethane and water (:20 by weight) was subjected topressure reduction in accordance with this invention. In the apparatusrepresented by the attached drawing, a 500 cc. autoclave equipped withan agitator was installed as the high-pressure vessel 1.Dichlorodifluoromethane (Freon- 12) was charged into the autoclave at arate of 2 kg./hr. by means of compressor 1.

At the same time, water was introduced into the high-pressure vessel 2at a rate of 500 cc./hr. by means of a plunger pump having a capacity of2 liter/hr. and a maximum operating pressure of 400 kg./cm. (similar tothe above-mentioned plunger pump 6 but not shown in the drawing). A 10percent by weight aqueous solution of methanol as the flow ratecontrolling medium was passed through a tube 2 mm. in diameter and 20 m.in length at a flow rate of 1.8 liter/min. by means of pump 6 so thatthe pressure indicated by Pl-2 might be kept at 50 kg./cm. When thepressure indicated by Pl-2 reacted 50 kg./cm. valve V was opened, theflow rate of the medium that was being supplied by means of pump 6 wasreduced to 1.7 liter/min, and thus pressure reduction of thedichlorodifluoromethane-water mixture in the autoclave was effected.

It is impossible to reduce the pressure of this mixture by using onlyreducing valves, since the water is frozen by the adiabatic expansion ofdichlorodifluoromethane.

TABLE 1 control of a Pressure-Reduction and Flow Rate Control ofNitrogen Initial pressure Gas flow rate of medium (kg/cm.G)(m.(NTP)/hr.)(l./min.)

TABLE 2 Pressure Reduction and Flow Rate Control of Ethylene Exam le 2 PFinal Pressure 0 lag/cm. G Medium Methanol Circuit 4 2 mm. 85 m. longtubing Circuit 2 mm. 20 m. Flow Rate Initial pressure Gas flow rate ofmedium H 'J (l.lmin.)

4O (PI-1.2) (Fl-1) (Fl-2) TABLE 3 Pressure Reduction and Flow RateControl of CO (Example 3 Final Pressure 0 kg./cm.* G Medium MethanolCircuit 4 2 mm. 85 m. long tubing Circuit 4:: 2 mm. 20 m. long tube FlowRate Initial pressure Gas Flow rate of medium (kg/cm." G) (m.(NTP)/hr.)(l./min.)

(914.2) (Fl-l) (Fl-2) We claim:

1. A method of reducing the pressure in a high-pressure fluid system andcontrolling the flow rate of the fluid in said system by passing thefluid through a pressure-reduction circuit, comprising the steps ofintroducing a liquid flow rate controlling medium into said circuit, themedium being in the liquid state under the temperature and pressureconditions in the circuit, and regulating the flow rate of the liquidmedium to reduce the flow rate of the fluid.

2. In the method of claim 1, the fluid being in a gaseous phase.

3. In the method of claim 1, the fluid being a two-phase gasliquidsystem.

4. In the method of claim 1 the fluid being a slurry.

5. In the method of claim 1, the fluid being in a liquid phaseconsisting of at least two immiscible liquids.

6. In the method of claim 1, the fluid being a gas-liquid-solid system.

7. In the method of claim 1, the liquid medium being immiscible with thefluid.

8. In the method of claim 1, the fluid being a compressible gas and thepressure-reduction circuit being a long tubing.

9. In the method of claim 1, the fluid being a liquefied gas and theliquid medium is miscible with the liquefied gas.

10. In the method of claim 1, the fluid being a slurry comprising aliquefied gas and a powder material, the liquid medium is miscible withthe liquefied gas, and the pressure-reduction circuit is a long tubing.

11. In the method of claim 9, the liquid medium being selected from thegroup consisting of water, methanol and mixtures thereof.

12. In the method of claim 11, the slurry comprising ethylene, methanoland polyethylene powder, and the liquid medium being methanol.

13. In the method of claim 1, the fluid being selected from the groupconsisting of ethylene, nitrogen, carbon dioxide, anethylene-polyethylene slurry, an ethylene-methanolpolyethylene powderslurry, and a dichlorodifluoromethanewater mixture.

14. In the method of claim 1, the fluid being a mixture of apolymerization product and unreacted residues being conducted through apressure-reducing circuit out of a high-pressure reactor in which atleast one monomer is polymerized under high pressure.

15. In the method of claim 14, wherein the mixture comprises thepolymerization product, at least one unreacted monomer, and apolymerization medium.

16. In the method of claim 14, wherein the polymerization product ispolyethylene.

"um; nncn

1. A method of reducing the pressure in a high-pressure fluid system andcontrolling the flow rate of the fluid in said system by passing thefluid through a pressure-reduction circuit, comprising the steps ofintroducing a liquid flow rate controlling medium into said circuit, themedium being in the liquid state under the temperature and pressureconditions in the circuit, and regulating the flow rate of the liquidmedium to reduce the flow rate of the fluid.
 2. In the method of claim1, the fluid being in a gaseous phase.
 3. In the method of claim 1, thefluid being a two-phase gas-liquid system.
 4. In the method of claim 1the fluid being a slurry.
 5. In the method of claim 1, the fluid beingin a liquid phase consisting of at least two immiscible liquids.
 6. Inthe method of claim 1, the fluid being a gas-liquid-solid system.
 7. Inthe method of claim 1, the liquid medium being immiscible with thefluid.
 8. In the method of claim 1, the fluid being a compressible gasand the pressure-reduction circuit being a long tubing.
 9. In the methodof claim 1, the fluid being a liquefied gas and the liquid medium ismiscible with the liquefied gas.
 10. In the method of claim 1, the fluidbeing a slurry comprising a liquefied gas and a powder material, theliquid medium is miscible with the liquefied gas, and thepressure-reduction circuit is a long tubing.
 11. In the method of claim9, the liquid medium being selected from the group consisting of water,methanol and mixtures thereof.
 12. In the method of claim 11, the slurrycomprising ethylene, methanol and polyethylene powder, and the liquidmedium being methanol.
 13. In the method of claim 1, the fluid beingselected from the group consisting of ethylene, nitrogen, carbondioxide, an ethylene-polyethylene slurry, anethylene-methanol-polyethylene powder slurry, and adichlorodifluoromethane-water mixture.
 14. In the method of claim 1, theflUid being a mixture of a polymerization product and unreacted residuesbeing conducted through a pressure-reducing circuit out of ahigh-pressure reactor in which at least one monomer is polymerized underhigh pressure.
 15. In the method of claim 14, wherein the mixturecomprises the polymerization product, at least one unreacted monomer,and a polymerization medium.
 16. In the method of claim 14, wherein thepolymerization product is polyethylene.